Synthetic bioepitope compounds which can be used as standards in the biological assays of Troponin I

ABSTRACT

The invention relates to synthetic biepitope compounds which can be used as standards in immunoassays, for the assay of troponin I, method for preparing them, compositions and kits containing such compounds as well as the immunoassay methods using such compounds.

This application is a 371 of PCT/FR97/02209 filed Dec. 4, 1997.

The present invention relates to synthetic biepitope compounds which can be used as standards in immunoassays, for the assay of troponin I, method for preparing them, compositions and kits containing such compounds as well as immunoassay methods using such compounds.

It is known that troponin is a myofibrillar protein complex consisting of three proteins, troponins I, T and C. This protein complex makes it possible to contribute to the regulation of muscle contraction by the Ca²⁺ ion by interacting with myosin and actin. More precisely, it is known that when a nerve impulse arrives at the level of the motor endplate of a muscle, there is generation of an action potential which is transmitted to the sarcoplasmic reticulum. Ca²⁺ is then released into the cytosol and binds to troponin C, which brings about a strengthening of the interaction between troponin I and troponin C and, consequently, a change in conformation of the troponin I, T, C complex. There is then liberation of the sites of actin-myosin interaction, which allows the muscle contraction movement.

When the muscle is damaged, whether it is the cardiac muscle, during myocardial necrosis following a myocardial infarction, or whether it is the skeletal muscle during sustained physical effort, the troponins then released appear more or less rapidly in the blood stream.

Thus, the assay of troponin has recently been recommended for the early diagnosis of myocardial infarction, whether that of troponin T in Circulation (1991,) 8, pp. 902-912, or of troponin I in Am. Heart J. (1987), 110, pp. 1333-1344, and Molecular Immunology (1992), 29 f 2), pp. 271-278. Likewise, the assay of cardiac troponin T for measuring the success of thrombolytic therapy following a myocardial infarction has been proposed in Br. Heart J., (1994), 7, pp. 242-248, as well as the assay of skeletal troponin I for the measurement of damage to the skeletal muscles (abstract No. 35 of the American Association for Clinical Chemistry, 46th National Meeting, New Orleans, Jul. 17-21, 1994). It should be noted that the assay of the various cardiac and skeletal troponins is nowadays a very useful means for the diagnosis of human and animal pathologies.

It is well known that the immunoassays performed in biological analysis laboratories require the supply by the manufacturer, besides the reagents necessary for the assay (that is to say antibodies, labelled or otherwise, revealing agents and diluting solutions), of a standard for the compound to be determined which, used under conditions similar to those of the sample to be studied, will serve as reference for calculating the results and/or as positive control.

To obtain the standard and/or the control for the compound to be determined, it is possible to use the said compound purified in freeze-dried form (accompanied by a solvent in which the compound will be dissolved by the user before use) or ready for use.

Because biological reagents are unstable, the standard or control solutions prepared from a freeze-dried product are frozen in unit doses and stored at −80° C. It has been observed, moreover, that these solutions were not stable for more than a few hours at +4° C., even if protease inhibitors or antibacterial agents were added to them. This therefore requires the users to prepare their standard solutions immediately before use.

The patent application published under the number FR-A-2,701,954 discloses a stabilized solution of troponin I or T for immunoassay, characterized in that it consists of an aqueous solution containing troponin I or troponin T, mixed with troponin C and particularly in proportions of 1 to 10 molar equivalents of troponin C per equivalent of troponin I or T, and calcium chloride. This technique allows preservation for several days at +4° C. of standard solutions, diluted to a greater or lesser extent, of troponin I or T.

The patent application published under the number FR 2,734,267 describes standard solutions of troponin composed of a ternary complex formed by troponin I, troponin T and troponin C.

The raw materials used to obtain these standards are of human or animal origin, and the standards or controls thus obtained are stable for about one month at +4° C.

Application WO 94/15217 describes some synthetic peptides useful as immunogens for the preparation of antibodies recognizing the N-terminal peptide of troponin I. Some of these peptides can be used as standards in immunoassays of troponin I, using the antibodies which are the subject of the invention covered by application WO 94/15217.

Likewise, patent application WO 94/27156 relates to a method of assaying cardiac troponin I, using antibodies specific for cardiac troponin I. These antibodies can be prepared from peptide fragments having a sequence absent from skeletal muscle troponin I and therefore specific for cardiac troponin I. However, this application neither discloses nor suggests the possibility of using certain peptide fragments as standards in immunoassays of troponin I.

It is also known that application WO 96/27661 describes aqueous solutions for stabilizing proteins and peptides. These solutions find application in particular in diagnostic tests for proteins or peptides.

According to WO 97/27661, these aqueous solutions even make it possible to increase the stability of the fragments of troponin I which are known to be less stable than whole troponin I.

Application EP-A-752 426 published on 8 Jan. 1997 also describes troponin I standards composed of one or more peptides bound to a carrier molecule such as high-molecular weight proteins (>100 KD) or polymers.

Application EP-A-650 053 describes synthetic standards containing active sites for one or more receptors, linked to each other with an arborescent structure. This application describes more particularly synthetic standards for troponin T which are stable in solution for only 3 weeks.

It has now been found that it is possible to obtain synthetic standards which can be used for the evaluation of troponin I, which are stable for several months.

Indeed, the specific structure of the compounds of the invention confer an excellent stability on them.

The compounds of the present invention therefore make it possible to obtain highly standardized standards, which are stable in solution for several months, and to avoid the purification steps and the complex extraction procedures which are necessary for the preparation of troponin I standards from animal organs.

The compounds of the invention comprise two epitopes of troponin I, linked to each other by a linker and correspond to the general formula I: Ξ-E₁-Z-E₂-Ψ  (I) in which:

-   -   E₁, and E₂, which are identical or different, represent a         peptide sequence comprising either a minimum epitope of troponin         I, or an extended epitope,     -   Z represents         -   a peptide sequence of 1 to 40 amino acids (aa), provided             however that -E₁-Z-E₂ - do not form together a portion of             the troponin I sequence,         -   a linear or branched amino(C₁-C₁₀ alkyl)carbonyl chain, or         -   a mixed construct consisting of at least one peptide             sequence of 1 to 10 amino acids and at least one linear or             branched amino(C₁-C₁₀ alkyl)carbonyl chain,     -   Ξ represents         -   a hydrogen atom, an acetyl group, a peptide sequence of 1 to             10 amino acids, an N-α-acetylated peptide sequence of 1 to             10 amino acids, a cysteinyl, biotinyl or biocytinyl group, a             peptide sequence of 1 to 10 amino acids carrying a             cysteinyl, biotinyl or biocytinyl residue,         -   a linear or branched amino(C₁-C₁₀ alkyl)carbonyl chain,         -   an N-α-acetylated linear or branched amino(C₁-C₁₀             alkyl)carbonyl chain,         -   a mixed construct consisting of at least one peptide             sequence of 1 to 10 amino acids and at least one linear or             branched amino(C₁-C₁₀ alkyl)carbonyl chain, or         -   a mixed construct consisting of at least one peptide             sequence of 1 to 10 amino acids and at least one linear or             branched amino(C₁-C₁₀ alkyl)carbonyl chain carrying a             biotinyl, biocytinyl or cysteinyl residue,     -   Ψ represents         -   a hydroxyl radical, an amino radical, a peptide sequence of             1 to 10 amino acids, a peptide sequence of 1 to 10 amino             acids carrying an amino-terminal group,         -   a linear or branched amino(C₁-C₁₀ alkyl)carbonyl chain,         -   a linear or branched amino(C₁-C₁₀ alkyl)carbonyl chain             carrying a hydroxyl radical or an amino radical,         -   a mixed construct consisting of at least one peptide             sequence of 1 to 10 amino acids and at least one linear or             branched amino(C₁-C₁₀ alkyl)carbonyl chain,         -   a mixed construct consisting of at least one peptide             sequence of 1 to 10 amino acids and at least one linear or             branched amino(C₁-C₁₀ alkyl)carbonyl chain carrying a             hydroxyl radical or an amino radical.

It is considered that -E₁-Z-E₂ - is not a portion of the troponin I sequence when -E₁-Z-E₂ - differs from a given fragment of the troponin I sequence by a nonconservative substitution, deletion or insertion of at least one amino acid, preferably of 2 amino acids, more particularly of 5 amino acids.

-   -   Z may in particular represent         -   a chain of formula II:             —NH—(CH₂)_(m)—CO—  (II)         -    in which m represents an integer from 1 to 10,         -   or a mixed construct of formulae III to V:             -[pep₁-NH—(CH₂)_(m)—CO]_(p)-  (III)             -[pep₁-NH—(CH₂)_(m)—CO-pep₂]_(p)-  (IV)             -[NH—(CH₂)_(m)—CO-pep₂]_(p)-  (V)         -    in which             -   m represents an integer from 1 to 10,             -   p represents an integer from 1 to 5, and             -   pep₁ and pep₂, which are identical or different,                 represent a peptide chain containing from 2 to 10 amino                 acids;     -   Ξ may in particular represent         -   a chain of formula IIa:             —NH—(CH₂)_(q)—CO—  (IIa)         -    in which q represents an integer from 1 to 10,         -   or a mixed construct of formulae IIIa to Va:             -[pep₃-NH—(CH₂)_(q)—CO]_(r)-  (IIIa)             -[pep₃-NH—(CH₂)_(q)—CO-pep₄]_(r)-  (IVa)             -[NH—(CH₂)_(q)—CO-pep₄]_(R)-  (Va)         -    in which             -   q represents an integer from 1 to 10,             -   r represents an integer from 1 to 5, and             -   pep₃ and pep₄, which are identical or different,                 represent a peptide chain containing from 2 to 10 amino                 acids,         -   an N-α-acetylated chain of formulae IIa to Va, or a chain of             formulae IIa to Va carrying a biotinyl, biocytinyl or             cysteinyl residue; and Ψ may in particular represent         -   a chain of formula IIb:             —NH—(CH₂)_(s)—CO—  (IIb)         -    in which s represents an integer from 1 to 10,         -   or a mixed construct of formulae IIIb to Vb:             -[pep₅-NH—(CH₂)_(s)—CO]_(t)-  (IIIb)             -[pep₅-NH—(CH₂)_(s)—CO-pep₆]_(t)-  (IVb)             -[NH—(CH₂)_(s)—CO-pep₆]_(t)-  (Vb)         -    in which             -   s represents an integer from 1 to 10,             -   t represents an integer from 1 to 5, and             -   pep₅ and pep₆, which are identical or different,                 represent a peptide chain containing from 2 to 10 amino                 acids,         -   a chain of formulae IIb to Vb carrying a hydroxyl radical or             an amino radical.

Among the preferred compounds of the invention are the compounds in which Ξ represents either a chain of formula IIa, or a mixed construct IIIa to Va, or an N-α-acetylated chain of formulae IIa to Va or a chain of formulae IIa to Va carrying a biotinyl, biocytinyl or cysteinyl residue.

Among the preferred compounds of the invention are also the compounds in which Ψ represents either a chain of formula IIb, or a mixed construct of formulae IIIb to Vb, or a chain of formulae IIb to Vb carrying a hydroxyl radical or an amino radical.

The compounds of the invention in which Z represents either a chain of formula II or a mixed construct of formulae III to V are preferred in particular.

The peptide sequences comprising a minimum epitope of troponin I have been determined with the aid of anti-troponin I monoclonal antibodies. Such antibodies are described by Larue et al. (Molec. Immunology, (1992), vol. 20 No. 29, pp. 271-278), Bodor et al. (Clin. Chem. (1992), vol 38 No. 11, pp. 2203-2214), Granier et al (Protein Science, (1997), vol. 6 suppl. 1. p. 61) and in application WO 94/15217. The majority of these antibodies are commercially available (Hytest LTD-Turku, Finland).

To determine the peptide sequence comprising a minimum epitope of troponin I, the simultaneous synthesis of peptides of 10 amino acids, in which the sequence overlaps with the sequence of the preceding peptide by 7 amino acids, was carried out according to the following scheme:

-   -   Peptide 1=amino acids 1-10     -   Peptide 2=amino acids 4-13     -   Peptide 3=amino acids 7-16     -   etc.

As a whole, the peptides synthesized and tested describe the entire sequence of the protein (Vallins J. et al., FEBS Letters (1990), vol. 270 No. 1-2, pp. 57-61).

The peptides which react with each of the monoclonal antibodies used were then identified by an immunoenzymatic test for binding of each peptide with each antibody.

Indicated for example below, for some anti-troponin I monoclonal antibodies, are the peptide sequence comprising a minimum epitope immunologically reactive for each antibody and the position of this peptide sequence in the troponin I sequence, as described in FEBS Letters (1990), vol. 270 No. 1-2, pp. 57-61.

Position in the troponin I Antibody Epitope* sequence 5C3 (SEQ ID NO:34) DAARE  7-11 10B11 (SEQ ID NO:3) PAPIRRR 16-22 HC11 (SEQ ID NO:35) RRRSS 20-24 HG5 and HC2 (SEQ ID NO:36) YRAYATEP 26-33 11E12 (SEQ ID NO:1) TEPH 31-34 10F4 (SEQ ID NO:2) HAKK 34-37 HC7 (SEQ ID NO:37) ISASRKLQLK 41-50 H414 (SEQ ID NO:38) AKQELE 57-62 5F1 (SEQ ID NO:39) AELQ 91-94 13H7 and 20C5 (SEQ ID NO:40) LQDLCR 93-98 2B7 (SEQ ID NO:41) DLTQKIFDLR 127-136 3G11 and 15F2 (SEQ ID NO:42) PTLRRVR 142-148 2E6 and 8G2 (SEQ ID NO:43) ADAMMQA 151-157 7D1 (SEQ ID NO:44) ALLG 157-160 8E1 (SEQ ID NO:4) LLGAR 158-162 9E8 (SEQ ID NO:45) SLDLRAH 166-172 4B5 (SEQ ID NO:46) ENRE 184-187 7F4 (SEQ ID NO:47) DWRKNID 190-196 *The peptide sequences are indicated using a single-letter code.

The peptide sequences derived from the said peptide sequences by substitution, deletion or insertion of an amino acid also belong to the field of the invention since they exhibit equivalent affinity for binding to the antibody.

The following linear sequences E₁ and/or E₂ No. 1 to 5 are preferred.

(SEQ ID NO:1): -Thr Glu Pro His- or -TEPH- (SEQ ID NO:2): -His Ala Lys Lys- or -HAKK- (SEQ ID NO:3): -Pro Ala Pro Ile Arg Arg Arg- or -PAPIRRR- (SEQ ID NO:4): -Leu Leu Gly Ala Arg- or -LLGAR- (SEQ ID NO:5): -Arg Lys Asn Ile- or -RKNI-.

The above sequences are given by way of a non-limiting example.

When the linker Z comprises a peptide sequence, the latter may form with the sequences E₁ and/or E₂ an extended epitope of troponin I. This extended epitope may in particular have a sequence (SEQ ID NOS 48-55 respectively, in order of appearance); -TEPHAKK- or -QALLGAR- or -PTLRRVRISA- or -GKFKRPTLRRVR- or -LGFAELQD- or -NYRAYATEPH- or -AYATEPH- or -LGFAELQ-, etc.

These extended epitopes, which amplify the immunological response for antibodies were determined by adding amino acids to the peptide sequence comprising a minimum epitope determined as described above.

As has been shown above, the peptide sequence of the linker Z may comprise from 1 to 40 amino acids. It is preferable that the number of amino acids which form the peptide sequence of Z is less than 30, more particularly less than 20.

A particularly preferred peptide sequence of Z includes a sequence selected from the following amino acid sequences(SEQ ID NOS 6, 8, 17, 22, 56, & 21, respectively, in order of appearance):

-   -   -Gln Lys Met Gln- or -QKMQ-,     -   -Gly Pro Asp Asn- or -GPDN-,     -   -Ala Met Met- or -AMM-,     -   -Ala Lys Lys- or -AKK-,     -   -Lys Ser Lys- or -KSK-, and     -   -Pro Gly Asn Ser- or -PGNS-.

This particularly preferred peptide sequence of Z may contain a repeated stretch of amino acids and has a number of amino acids less than 30.

By way of example, there are given the Z peptide sequences of the following formulae in which n represents an integer from 1 to 5:

(SEQ ID NO:6): -Gln Lys Met Gln- or -QKMQ- (SEQ ID NO:7): -Gln Gly Pro Asp Asn- or -QGPDN- (SEQ ID NO:8): -Gly Pro Asp Asn- or -GPDN- (SEQ ID NO:9): Ala Met Met Lys Ser Lys (Gln Lys Met Gln)_(n)- or -AMMKSK-(QKMQ)_(n)- (SEQ ID NO:10): -Ala Lys Lys Ala Met Met Lys Ser Lys-(Gln Lys Met Gln)_(n)- or -AKKAMMKSK-(QKMQ)_(n)- (SEQ ID NO:11): -Ala Met Met Lys Ser Lys-(Gln Lys Met Gln)_(n)-Gln Ala- or -AMMKSK-(QKMQ)_(n)-QA- (SEQ ID NO:12): -Ala Lys Lys Ala Met Met Lys Ser Lys-(Gln Lys Met Gln)_(n)-Gln Ala- or -AKKAMMKSK-(QKMQ)_(n)-QA- (SEQ ID NO:13): -Lys Ser Lys-(Gln Lys Met Gln)_(n)-Ala Met Met- or -KSK-(QKMQ)_(n)-AMM- (SEQ ID NO:14): -Ala Lys Lys Lys Ser Lys-(Gln Lys Met Gln)_(n)-Ala Met Met- or -AKKKSK-(QKMQ)_(n)-AMM- (SEQ ID NO:15): -Lys Ser Lys-(Gln Lys Met Gln)_(n)-Ala Met Met Gln Ala- or -KSK-(QKMQ)_(n)-AMMQA- (SEQ ID NO:16): -Ala Lys Lys Lys Ser Lys-(Gln Lys Met Gln)_(n)-Ala Met Met Gln Ala- or -AKKKSK-(QKMQ)_(n)-AMMQA- (SEQ ID NO:17): -Ala Met Met- or -AMM- (SEQ ID NO:18): -Ala Lys Lys Ala Met Met- or -AKKAMM- (SEQ ID NO:19): -Ala Met Met Gln Ala- or -AMMQA- (SEQ ID NO:20): -Ala Lys Lys Ala Met Met Gln Ala- or - AKKAMMQA- (SEQ ID NO:21): -Pro Gly Asn Ser- or -PGNS-

Z may also represent a mixed construct of formulae III to V, in which m is an integer from 1 to 10.

By way of a non-limiting example, Z may thus correspond to the following sequences:

(SEQ ID NO:22) -Ala Lys Lys-NH-(CH₂)_(m)-CO- or -AKK-NH-(CH₂)_(m)-CO- (SEQ ID NO:23) -NH-(CH₂)_(m)-CO-Ala Met Met- or -NH-(CH₂)_(m)-CO-AMM- (SEQ ID NO:24) -Ala Lys Lys-NH-(CH₂)_(m)-CO-Ala Met Met- or -AKK-NH-(CH₂)_(m)-CO-AMM- (SEQ ID NO:25) -NH-(CH₂)_(m)-CO-Ala Met Met Gln Ala- or -NH-(CH₂)_(m)-CO-AMMQA- (SEQ ID NO:26) -Ala Lys Lys-NH-(CH₂)_(m)-CO-Ala Met Met Gln Ala- or -AKK-NH-(CH₂)_(m)-CO-AMMQA-

Preferred compounds of the present invention are the biepitope compounds of formula I, in which Z includes a sequence selected from the sequences No. 6 to 26, as defined above.

When Ξ and/or Ψ contain a peptide sequence, this sequence may form an extended epitope with E₁ and/or E₂ with which it is linked.

Advantageously, the chemical structure of Ξ is such that it allows the binding of the biepitope compounds of the invention to a natural carrier protein, to a peptide construct or to a solid phase.

The synthetic biepitope peptides of formula Ia and Ib form part of the peptides of formula I and are preferred peptides: (These formula peptides are encompassed by various peptide sequences listed throughout the application, including those peptides disclosed in the tables shown on page 14.); Ξ-Thr Glu Pro His-Z-Leu Leu Gly Ala Arg-Ψ  (Ia) Ξ-Pro Ala Pro IIe Arg Arg Arg-Z-Thr Glu Pro His-Ψ  (Ib)

In the formulae Ia and Ib:

-   -   Z includes or represents a sequence selected from the following         sequences:         (SEQ ID NO: 6): -Gln Lys Met Gin-         (SEQ ID NO: 7): -Gln Gly Pro Asp Asn-         (SEQ ID NO: 8): -Gly Pro Asp Asn-         (SEQ ID NO: 20): -Ala Met Met Gln Ala-         (SEQ ID NO: 26): -Ala Lys Lys-NH-(CH₂)_(m)-CO-Ala Met Met Gln         Ala         (SEQ ID NO: 12): -Ala Lys Lys Ala Met Met Lys Ser Lys-(Gln Lys         Met Gln)_(n)-Gln Ala-         (SEQ ID NO: 16): -Ala Lys Lys Lys Ser Lys-(Gln Lys Met Gln)n-Ala         Met Met Gln Ala-         (SEQ ID NO: 21): -Pro Gly Asn Ser-,         in which n represents an integer from 1 to 5 and m an integer         from 1 to 10,     -   Ξ represents an acetyl radical, a cysteinyl radical, a biotinyl         or biocytinyl radical optionally linked with the residue of a         peptide sequence comprising a sequence selected from the         following sequences: (SEQ ID NOS 57-60, respectively,in order of         appearance);         -   -Gly Asn Tyr Arg Ala Tyr Ala-         -   -Gly Gly Asn Tyr Arg Ala Tyr Ala-         -   -Asn Tyr Arg Ala Tyr Ala-, and         -   -Arg Pro Ala-             and     -   Ψ represents an amino radical or a peptide sequence comprising         one of the following sequences: (SEQ ID NOS 61-62, respectively,         in order of appearance);         -   -Ala Lys Glu         -   -Ala Lys Lys Lys Ser Lys.

Any peptide including one of the sequences 27 to 33, which are the following, is preferred:

(SEQ ID NO:27) -Gly Asn Tyr Arg Ala Tyr Ala Thr Glu Pro His Ala Lys Lys Ala Met Met Gln Ala Leu    1               5                  10                  15                  20 Leu Gly Ala Arg Ala Lys Glu-                  25 (SEQ ID NO:28) -Gly Asn Tyr Arg Ala Tyr Ala Thr Glu Pro His Ala Lys Lys -NH-(CH₂)₅-CO-Ala Met    1               5                  10                       15           17 Met Gln Ala Leu Leu Gly Ala Arg Ala Lys Glu-          20                      25 (SEQ ID NO:29) -Asn Tyr Arg Ala Tyr Ala Thr Glu Pro His Ala Lys Lys Lys Ser Lys Gln Lys Met Gln    1               5                  10                  15                  20 Ala Met Met Gln Ala Leu Leu Gly Ala Arg Ala Lys Glu-                  25                  30 (SEQ ID NO:30) -Gly Gly Asn Tyr Arg Ala Tyr Ala Thr Glu Pro His Ala Lys Lys Lys Ser Lys Gln Lys    1               5                  10                  15                  20 Met Gln Ala Met Met Gln Ala Leu Leu Gly Ala Arg Ala Lys Glu-                  25                  30                  35 (SEQ ID NO:31) -Gly Gly Asn Tyr Arg Ala Tyr Ala Thr Glu Pro His Ala Lys Lys Lys Ser Lys Gln Lys    1               5                  10                  15                  20 Met Gln Gln Lys Met Gln Ala Met Met Gln Ala Leu Leu Gly Ala Arg Ala Lys Glu-                  25                  30                  35 (SEQ ID NO:32) -Gly Asn Tyr Arg Ala Tyr Ala Thr Glu Pro His Ala Lys Lys Lys Ser Lys Gln Lys Met    1               5                  10                  15                  20 Gln Gln Lys Met Gln Ala Met Met Gln Ala Leu Leu Gly Ala Arg Ala Lys Glu-                  25                  30                  35 (SEQ ID NO:33) -Arg Pro Ala Pro Ala Pro Ile Arg Arg Arg Pro Gly Asn Ser Thr Glu Pro His Ala Lys    1               5                  10                  15                  20 Lys Lys Ser Lys-

The synthetic biepitope peptides of formula Ia and Ib below are particularly preferred peptides. In the two tables which follow, the peptide sequences are given using the single-letter code.

Ξ-TEPH-Z-LLGAR-Ψ (Ia) COMPOUND Ξ- -Z- -Ψ 1 (SEQ ID NO:6) Ac- -QKMQ- -NH₂ 2 (SEQ ID NO:8) Ac- -GPDN- -NH₂ 3 (SEQ ID NO:6) biotinyl- -QKMQ- -NH₂ 4 (SEQ ID NO:8) biotinyl- -GPDN- -NH₂ 5 (SEQ ID NO:64) Ac-GNYRAYA- -AKKAMMQA- -AKE-NH₂ 6 (SEQ ID NO:65) Ac-GNYRAYA- -AKK-NH-(CH₂)₅-CO-AMMQA- -AKE-NH₂ 7 (SEQ ID NO:66) Ac-GGNYRAYA- -AKKAMMKSK-(QKMQ)₂-QA- -AKE-NH₂ 8 (SEQ ID NO:67) Ac-GGNYRAYA- -AKKAMMKSK-(QKMQ)₁-QA- -AKE-NH₂ 9 (SEQ ID NO:68) Ac-NYRAYA- -AKKAMMKSK-(QKMQ)₁-QA- -AKE-NH₂ 10 (SEQ ID NO:69) Ac-NYRAYA- -AKKKSK-(QKMQ)₁-AMMQA- -AKE-NH₂ 11 (SEQ ID NO:70) Ac-GGNYRAYA- -AKKKSK-(QKMQ)₁-AMMQA- -AKE-NH₂ 12 (SEQ ID NO:71) Ac-GGNYRAYA- -AKKKSK-(QKMQ)₂-AMMQA- -AKE-NH₂ 13 (SEQ ID NO:72) Ac-GNYRAYA- -AKKKSK-(QKMQ)₂-AMMQA- -AKE-NH₂ 14 (SEQ ID NO:73) CGNYRAYA- -AKK-NH-(CH₂)₅-CO-AMMQA- -AKE-NH₂

Ξ-PAPIRRR-Z-TEPH-Ψ (Ib) COMPOUND Ξ- -Z- -Ψ 15 (SEQ ID NO:74) Ac-RPA -PGNS- -AKKKSK-NH₂

In the formulae Ia and Ib, the underlined amino acid sequences, corresponding to E₁ and E₂, represent a minimum epitope of troponin I. The underlined amino acid sequences of Z form with E₁ or E₂, to which they are linked, an extended epitope of troponin I.

Among the compounds indicated above, compounds 5, 6, 10, 11, 12, 13, 14 and 15 are particularly preferred compounds.

These peptides have the following sequences:

(SEQ ID NO:27) Ac-Gly Asn Tyr Arg Ala Tyr Ala Thr Glu Pro His Ala Lys Lys Ala Met Met Gln Ala Leu      1               5                  10                  15                  20 Leu Gly Ala Arg Ala Lys Glu-NH₂                  25 (SEQ ID NO:28) Ac-Gly Asn Tyr Arg Ala Tyr Ala Thr Glu Pro His Ala Lys Lys -NH-(CH₂)₅-CO-Ala Met      1               5                  10                       15 Met Gln Ala Leu Leu Gly Ala Arg Ala Lys Glu-NH₂          20                  25 (SEQ ID NO:29) Ac-Asn Tyr Arg Ala Tyr Ala Thr Glu Pro His Ala Lys Lys Lys Ser Lys Gln Lys Met Gln      1               5                  10                  15                  20 Ala Met Met Gln Ala Leu Leu Gly Ala Arg Ala Lys Glu-NH₂                  25                  30 (SEQ ID NO:30) Ac-Gly Gly Asn Tyr Arg Ala Tyr Ala Thr Glu Pro His Ala Lys Lys Lys Ser Lys Gln Lys      1               5                  10                  15                  20 Met Gln Ala Met Met Gln Ala Leu Leu Gly Ala Arg Ala Lys Glu-NH₂                  25                  30                  35 (SEQ ID NO:31) Ac-Gly Gly Asn Tyr Arg Ala Tyr Ala Thr Glu Pro His Ala Lys Lys Lys Ser Lys Gln Lys      1               5                  10                  15                  20 Met Gln Gln Lys Met Gln Ala Met Met Gln Ala Leu Leu Gly Ala Arg Ala Lys Glu-NH₂                  25                  30                  35 (SEQ ID NO:32) Ac-Gly Asn Tyr Arg Ala Tyr Ala Thr Glu Pro His Ala Lys Lys Lys Ser Lys Gln Lys Met      1               5                  10                  15                  20 Gln Gln Lys Met Gln Ala Met Met Gln Ala Leu Leu Gly Ala Arg Ala Lys Glu-NH₂                  25                  30                  35 (SEQ ID NO:63) Cys-Gly Asn Tyr Arg Ala Tyr Ala Thr Glu Pro His Ala Lys Lys -NH-(CH₂)₅-CO-Ala Met       1               5                  10                      15 Met Gln Ala Leu Leu Gly Ala Arg Ala Lys Glu-NH₂          20                  25 (SEQ ID NO:33) Ac-Arg Pro Ala Pro Ala Pro Ile Arg Arg Arg Pro Gly Asn Ser Thr Glu Pro His Ala Lys      1               5                  10                  15                  20 Lys Lys Ser Lys-NH₂

The synthetic biepitope compounds of formula I, which are the subject of the present invention, are obtained by solid phase synthesis according to conventional methods: R. B. Merrifield, J. Amer. Chem. Soc. (1963), 85, pp. 2149-2154; R. C. Sheppard, in “Peptides 1971”, Nesvadba H (ed.) North Holland, Amsterdam, pp. 111; E. Atherton and R. L. Sheppard, in “Solid phase peptide synthesis, a practical approach”, IRL PRESS, (1989), Oxford University Press, pp. 25-34. As automatic synthesizer, the Millipore 9050 Plus Pep Synthesizer or an equivalent synthesizer can be used.

The solid support used for the syntheses should be compatible with the technique and the chemistry used. For example, for synthesis on the 9050 Plus Pep Synthesizer, it is recommended to use a resin suited to the so-called “continuous flow” technique; the PEG PS resins meet these criteria. These supports consist of a spacer based on polyethylene glycol (PEG) situated between the functional group of the polystyrene of the beads and the point of attachment of the first amino acid. The nature of this point of anchorage may vary according to the C-terminal functional group selected. For example, for a peptide in the form of an amide, a PAL PEG PS type resin may be taken.

The starting resin and the amino acids used as raw material are products which are commercially available (PerSeptive-Biosystem).

The following side chain protecting groups were used:

Amino acids Protecting groups Arginine 2,2,4,6,7-pentamethyl-5-dihydrobenzofuran- sulphonyl (Pbf) Asparagine, Glutamine Trityl (Trt) Glutamic acid Tert-butyl ester (otBu) Threonine, Tyrosine Tert-butyl ether (tBu) Lysine Tert-butyloxycarbonyl (Boc)

The primary amine functional group was temporarily protected at the α position of the amino acids with the aid of the 9-fluorenylmethyloxycarbonyl (Fmoc) group. The deprotection is carried out with a 20% piperidine solution in dimethylformamide.

For the coupling, an excess of diisopropylcarbodiimide (DIPCDI) and of 1-hydroxybenzotriazole (HOBt) is preferably used.

After synthesis, the resin is washed with organic solvents (dimethylformamide and then dichloromethane), dried under vacuum and then treated with a trifluoroacetic acid-based solution cooled to 0° C. and containing appropriate scavengers. It will be possible to use, for example, the K reagent containing 82% trifluoroacetic acid, 5% phenol, 5% water, 5% thioanisole and 3% ethanedithiol.

The synthetic biepitope peptides thus isolated are then precipitated and rinsed with ether.

The synthetic biepitope compounds are then purified by reversed-phase liquid chromatography and their purity is determined by mass spectrometry. The Bondapak C₁₈ phase can be used, for example, as phase. The peptides are eluted by forming a linear gradient between two buffer solutions, the first of which is essentially aqueous (for example water-TFA 0.1%) and the second of which is rather organic (for example a mixture containing 60% acetonitrile, 40% water and 0.08% TFA). The pure fractions collected are combined, concentrated under vacuum and freeze-dried.

The biepitope character of the compounds of formula I was determined using a troponin I assay kit. It is possible to use, for example, a kit allowing a sandwich-type immunoassay to be used and containing monoclonal antibodies which react with the peptide sequences E₁ and E₂.

To determine the biepitope character, the compounds of formula I are diluted in the diluent of the kit, which may be for example normal human serum or a buffer solution, and they are assayed like a patient's sample.

The use of compounds of formula I as standard for the immunological assay of troponin I also forms part of the invention.

The present invention also relates to compositions containing the compounds of formula I. They are preferably aqueous solutions or compositions consisting of compounds of formula I in a buffer solution. As buffer solution, it is possible to use, for example, a phosphate buffer solution (KH₂PO₄/K₂HPO₄ pH=6.5-7.5) containing Kathon and BSA or Kathon, Régilait and EDTA, or Kathon, Plasmion and optionally EDTA, or Kathon, casein and EDTA.

It is also possible to use succinate buffer (pH=5-6) or Tris-HCl buffer (pH=7.5-8.5) solutions containing Kathon and BSA or Kathon, Régilait and EDTA or Kathon, Plasmion and optionally EDTA, or Kathon, casein and EDTA.

Buffer solutions containing glycine, Kathon, Régilait and EDTA can also be used.

Succinate or phosphate buffer solutions containing Kathon, Régilait and EDTA are preferred.

Kathon®, an antibacterial agent marketed by the company Rhom and Haas, consists of 5-chloro-2-methyl4-isothiazolin-3-one and 2-methyl4-isothiazolin-3-one (1.5%).

Régilait® is marketed by the company Régilait.

Plasmion® is marketed by Bellon Laboratories and consists of modified fluid gelatin (30 g/l), NaCl (5.382 g/l), MgCl (143 mg/l), KCl (373 mg/l), sodium lactate (3.360 g/l), in water.

The following buffer solutions are particularly preferred:

-   -   0.1 M succinate buffer solution (pH=6) containing Kathon (0.2%),         Régilait (0.05-2%) and 2 mM EDTA,     -   0.1 M succinate buffer solution (pH=6) containing Kathon (0.2%),         casein (0.01-0.5%) and 2 mM EDTA,     -   0.1 M phosphate butter solution (KH₂PO₄/K₂HPO₄) (pH=7.5)         containing Kathon (0.2%), Régilait (0.05-0.5%) and 2 mM EDTA,     -   0.1 M phosphate buffer solution (KH₂PO₄/K₂PO₄) (pH=7.5)         containing Kathon (0.2%), casein (0.01-0.1%) and 2 mM EDTA.

Compositions containing plasma and a compound of formula I also form part of the present invention.

Immunoassay procedures using, as standards or controls, the compounds of formula I also form part of the invention.

The invention also relates to kits for carrying out immunoassays which include a compound of formula I or a composition which contains a biepitope peptide of formula I.

The following examples illustrate the invention and are given with no limitation being implied.

EXAMPLE 1 Preparation of a Compound According to the Invention (SEQ ID NO: 29).

Ac-Asn Tyr Arg Ala Tyr Ala Thr Glu Pro His Ala Lys Lys Lys Ser Lys Gln Lys Met Gln      1               5                  10                  15                  20 Ala Met Met Gln Ala Leu Leu Gly Ala Arg Ala Lys Glu-NH₂                  25                  30

This peptide was synthesized on a solid phase. The technique developed in 1963 by Merrifield (J. Am. Chem. Soc. (1963), pp. 2149-2154) consists in attaching the first amino acid onto a polymeric solid support (resin) by its acid functional group and in extending the peptide sequence from this first amino acid, the peptide being synthesized remaining anchored onto the resin.

For the synthesis of compound 10, the 9050 Plus Pep Synthesizer was used as synthesizer and the PEG PS resin described above, as resin.

The various steps of the synthesis are summarized in Table I:

TABLE I No. of eq- AMINOACID NH₂ SIDE METHOD OF DURATION RESIDUE PROTECTION PROTECTION COUPLING (DC) Glu Fmoc otBu DIPCDI/HOBt 5 eq - 30 min Lys Fmoc Boc DIPCDI/HOBt 5 eq - 30 min Ala Fmoc DIPCDI/HOBt 5 eq - 30 min Arg Fmoc Pbf DIPCDI/HOBt 5 eq - 30 min Ala Fmoc DIPCDI/HOBt 5 eq - 30 min Gly Fmoc DIPCDI/HOBt 5 eq - 30 min Leu Fmoc DIPCDI/HOBt 5 eq - 30 min Leu Fmoc DIPCDI/HOBt 5 eq - 30 min Ala Fmoc DIPCDI/HOBt 5 eq - 30 min Gln Fmoc Trt DIPCDI/HOBt 5 eq - 30 min Met Fmoc DIPCDI/HOBt 5 eq - 30 min Met Fmoc DIPCDI/HOBt 5 eq - 30 min Ala Fmoc DIPCDI/HOBt 5 eq - 30 min Gln Fmoc Trt DIPCDI/HOBt 5 eq - 30 min Met Fmoc DIPCDI/HOBt 5 eq - 30 min Lys Fmoc Boc DIPCDI/HOBt 5 eq - 30 min Gln Fmoc Trt DIPCDI/HOBt 5 eq - 30 min Lys Fmoc Boc DIPCDI/HOBt 5 eq - 30 min Ser Fmoc tBu DIPCDI/HOBt 5 eq - 30 min Lys Fmoc Boc DIPCDI/HOBt 5 eq - 30 min Lys Fmoc Boc DIPCDI/HOBt 5 eq - 30 min Lys Fmoc Boc DIPCDI/HOBt 5 eq - 30 min Ala Fmoc DIPCDI/HOBt 5 eq - 30 min His Fmoc Trt DIPCDI/HOBt 5 eq - 30 min Pro Fmoc DIPCDI/HOBt 5 eq - 30 min Glu Fmoc otBu DIPCDI/HOBt 5 eq - 30 min Thr Fmoc tBu DIPCDI/HOBt 5 eq - 30 min Ala Fmoc DIPCDI/HOBt 5 eq - 30 min Tyr Fmoc tBu DIPCDI/HOBt 5 eq - 30 min Ala Fmoc DIPCDI/HOBt 5 eq - 30 min Arg Fmoc Pbf DIPCDI/HOBt 5 eq - 30 min Tyr Fmoc tBu DIPCDI/HOBt 5 eq - 30 min Asn Fmoc Trt DIPCDI/HOBt 5 eq - 30 min Acetyl (CH₃CO)₂O/DIEA 5 eq - 30 min

At the end of the synthesis, the resin was washed with dimethylformamide, then with dichloromethane and dried under vacuum.

Next, the resin was treated with K reagent (82% trifluoroacetic acid; 5% phenol; 5% water; 5% thioanisole; 3% ethanedithiol). Compound 10 thus isolated by precipitation was then rinsed with diethyl ether. 0.529 g of compound 10 was thus obtained.

In a similar manner, and using the appropriate amino acids, the other compounds of the invention were synthesized.

The molecular weight, evaluated by mass spectrometry, of some compounds of formula Ia and Ib are given below.

Molecular weight Compound (Dalton) 1 1549.8 2 1545.5 5 3018 6 3132.5 7 4448 8 3934.5 9 3820.5 10 3820.5 11 3934.5 12 4448 13 4391 15 2721

EXAMPLE 2 Evaluation of the Immunoreactivity of the Peptides According to the Invention

The biepitope character of the compounds mentioned above was determined using a troponin I assay kit. It is the kit ERIA Troponin I Pasteur Code 79691.

Its principle is based on a sandwich type immunoenzymatic method which allows a quantitative assay of cardiac troponin I in human serum.

The solid phase consists of polystyrene tubes coated with an anti-cardiac troponin I monoclonal antibody (8E1).

The revealing is performed with the aid of a second anti-cardiac troponin I monoclonal antibody (11E12) coupled to peroxidase.

The carrying out of the test comprises the following steps:

The samples and the standards (troponin I standards of animal origin) as well as the peroxidase-coupled monoclonal antibody are incubated in the presence of the anti-cardiac troponin I monoclonal antibody immobilized on the solid phase.

After a series of washes, the enzymatic revealing is carried out by adding the tetramethylbenzidine chromogen.

After stopping the revealing, the optical density is read at λ=450 nm. The absorbance obtained is directly correlated with the concentration of cardiac troponin I present in a tube.

Compounds 1, 2, 5, 6, 10, 11 and 12 were dissolved in water (C=1 mg/ml or 2 mg/ml depending on the case). These solutions were rediluted using a 0.1 M succinate buffer pH=6 containing 0.05% Régilait and 0.2% Kathon and 2 mM EDTA. The “final solutions” were then assayed as “samples” according to the protocol described above.

Moreover, compounds 1, 2, 10 and 11 were tested after dilution in human serum. In this case, the “final solutions” were obtained by diluting, in normal human serum, aqueous “stock” solutions containing compounds 1, 2, 10 and 11 at a concentration of 2 mg/ml.

All the compounds are reactive in the assay of troponin I.

EXAMPLE 3 Tests of Stability

A compound according to the invention (compound 10) was diluted in water (C=2 mg/ml). From this “stock” solution, four “working” solutions S₁, S₂, S₃ and S₄ are obtained.

To carry out the various dilutions, a 0.1 M sodium succinate buffer solution (pH=6) containing Kathon (0.2%), Re gilait (0.05%) and 2 mM EDTA was used for solutions S₁ (C=0.5 ng/ml) and S₂ (C=1 ng/ml).

Solutions S₃ (C=0.5 ng/ml) and S₄ (C=1 ng/ml) were obtained by diluting the “stock” solution with phosphate buffer (KH₂PO4/K₂HPO₄ pH=7.5) containing Kathon (0.2%), Régilait (0.05%) and 2 mM EDTA.

Solutions S₁, S₂, S₃ and S₄ were assayed, according to the protocol described in Example 2, on D0 (day of preparation of the solutions). The values obtained served as reference for monitoring stability.

Next, solutions S₁, S₂, S₃ and S₄ were stored at +4° C. and assayed periodically. During each series of assays, three “control solutions” were used. They are the freeze-dried troponin I control sera distributed in the assay measuring range. For these sera, it was demonstrated beforehand that the storage life of a freeze-dried control serum is greater than 18 months. During each stability test, it was checked that each of the controls tested was obtained at its target concentration. This therefore made it possible to compare the concentrations obtained for the peptides during the various tests. In particular, it was possible to compare the concentration of the solution of the compound according to the invention at D0 (day on which the solution was made) with the concentration of the compound according to the invention during the stability trials over time. The fact that, for the controls, a constant value is always found during the stability trials makes it possible to validate the tests carried out.

The results obtained during the stability trials are indicated in Tables II and III.

TABLE II Absorbance (λ = 450 nm) D + 2 D + 3 D + 4 Solutions D0 D + 1 month months months months Blank (C = 0 ng/ml) 0.00 0.00 0.01 0.00 0.03 S₁ (C = 0.5 ng/ml) 0.42 0.038 0.44 0.40 0.40 S₂ (C = 1.0 ng/ml) 1.05 0.90 0.87 0.95 1.10 Absorbance (λ = 450 nm) D + 4 D + 1 D + 2 D + 3 months/ Solutions month/D0 months/D0 months/D0 D0 S₁ (C = 0.5 ng/ml) 0.90 1.05 0.95 0.95 S₂ (C = 1.0 ng/ml) 0.86 0.83 0.90 1.05

TABLE III Absorbance (λ = 450 nm) D + 2 D + 3 D + 4 Solutions D0 D + 1 month months months months Blank (C = 0 ng/ml) 0.00 0.00 0.00 0.00 0.00 S₃ (C = 0.5 ng/ml) 0.27 0.23 0.27 0.25 0.28 S₄ (C = 1.0 ng/ml) 0.71 0.59 0.68 0.63 0.63 D + 4 D + 1 D + 2 D + 3 months/ month/D0 months/D0 months/D0 D0 S₃ (C = 0.5 ng/ml) 0.85 1.00 0.93 1.04 S₄ (C = 1.0 ng/ml) 0.83 0.96 0.89 0.89

The results indicated in Tables I and II demonstrate the excellent stability of the solutions of compound 10 according to the invention.

Trials carried out with other compounds of formula I demonstrated that their stability is comparable to that of compound 10. For example, the trials carried out with compound 6 confirmed that this compound had an excellent stability for at least 9 months, when stored at +4° C. in solution in a 0.1 M sodium succinate buffer solution (pH=6) containing Kathon (0.2%), Régilait (0.2%) and 2 mM EDTA. Compound 6 was dissolved in the buffer indicated above in order to obtain concentrations of 0.25, 4.8 and 18 ng/ml.

This stability study was validated with the aid of compound 6 stored at −20° C. and used as reference.

It was checked beforehand that compound 6 is stable when stored at −20° C. for at least 18 months. The results of this trial are given in Table IV.

TABLE IV Concentration in ng/ml D + 1 D + 2 D + 3 D + 4 D + 5 D + 6 J + 9 Solutions D0 month months months months months months months Solution 1 0.26 0.29 0.24 0.24 0.23 0.25 0.23 0.22 Solution 2 4.93 5.25 4.58 4.77 4.57 4.42 — 3.87 Solution 3 18.21 19.61 16.79 17.48 16.35 17.21 16.13 15.04 

1. Biepitope compound consisting of formula I: Ξ-E₁-Z-E₂-Ψ  (I) in which: E₁ and E2, which are identical or different, represent a peptide sequence comprising a minimum epitope of troponin I, Z represents a chain of formula II: —NH—(CH₂)_(m)—CO—  (II)  in which m represents an integer from 1 to 10, or a mixed construct of formulae III to V: -[pep₁-NH—(CH₂)_(m)—CO]_(p)-  (III) -[pep₁-NH—(CH₂)_(m)—CO-pep₂]_(p)-  (IV) -[NH—(CH₂)_(m)—CO-pep2]_(p)-  (V)  in which m represents an integer from 1 to 10, p represents an integer from 1 to 5, and pep₁ and pep₂ which are identical or different, represent a peptide chain containing from 2 to 10 amino acids forming an extended epitope of troponin I with the sequences E₁ an E₂, respectively, Ξ represents a hydrogen atom, an acetyl group, a peptide sequence of 1 to 10 amino acids, an N-α-acetylated peptide sequence of 1 to 10 amino acids, a cysteinyl, biotinyl or biocytinyl group, a peptide sequence of 1 to 10 amino acids carrying a cysteinyl, biotinyl or biocytinyl residue, a linear or branched amino(C₁-C₁₀ alkyl)carbonyl chain, an N-α-acetylated linear or branched amino(C₁-C₁₀ alkyl)carbonyl chain, a mixed construct of at least one peptide sequence of 1 to 10 amino acids and at least one linear or branched amino(C₁-C₁₀ alkyl)carbonyl chain, or a mixed construct of at least one peptide sequence of 10 to 10 amino acids and at least one linear or branched amino(C₁-C₁₀ alkyl)carbonyl chain carrying a cysteinyl, biotinyl, or biocytinyl residue, Ψ represents a hydroxyl radical, an amino radical, a peptide sequence of 1 to 10 amino acids, or a peptide sequence of 1 to 10 amino acids carrying an amino-terminal group, a linear or branched amino(C₁-C₁₀ alkyl)carbonyl chain, a linear or branched amino(C₁-C₁₀ alkyl)carbonyl chain carrying a hydroxyl radical or an amino radical, a mixed construct of at least one peptide sequence of 1 to 10 amino acids and at least one linear or branched amino(C₁-C₁₀ alkyl)carbonyl chain, or a mixed construct of at least one peptide sequence of 1 to 10 amino acids and at least one linear or branched amino(C₁-C₁₀ alkyl)carbonyl chain carrying a hydroxyl radical or an amino radical.
 2. The biepitope compound according to claim 1, in which E₁ and/or E₂, which are identical or different, represent a peptide sequence selected from the group consisting of SEQ ID NO: 1 -Thr Glu Pro His-, SEQ ID NO: 2 -His Ala Lys Lys-, SEQ ID NO: 3 -Pro Ala Pro IIe Arg Arg Arg- SEQ ID NO: 4 -Leu Leu Gly Ala Arg, and SEQ ID NO: 5 -Arg Lys Asn IIe-.
 3. The biepitope compound according to claim 1 of formulae Ia or Ib: Ξ-Thr Glu Pro His-Z-Leu Leu Gly Ala Arg-Ψ  (Ia) Ξ-Pro Ala Pro IIe Arg Arg Arg-Z-Thr Glu Pro His-Ψ  (Ib) in which Z includes sequence SEQ ID NO: 26 -Ala Lys Lys-NH—(CH₂)_(m)—CO-Ala Met Met Gln Ala-, wherein m represents an integer from 1 to 10, Ξ represents an acetyl radical, a cysteinyl radical, or a biotinyl or biocytinyl radical optionally linked with a peptide sequence residue selected from the group consisting of: SEQ ID NO: 57 -Gly A Tyr Arg Ala Tyr Ala-, SEQ ID NO: 58 -Gly Gly Asn Tyr Arg Ala Tyr Ala-, SEQ ID NO: 59 -Asn Tyr Arg Ala Tyr Ala-, and SEQ ID NO: 60 -Arg Pro Ala- and Ψ represents an amino radical or a peptide sequence selected from the group consisting of SEQ ID NO: 61 -Ala Lys Glu and SEQ ID NO: 62 -Ala Lys Lys Lys Ser Lys.
 4. The biepitope compound according to claim 1, selected from the group consisting of Ac-Gly Asn Tyr Arg Ala Tyr Ala Thr Glu Pro His Ala Lys Lys-NH(CH₂)₅—CO-ala Met      1              5                   10                      15 Met Gin Ala Leu Leu Gly Ala Arg Ala Lys Glu-NH₂ (SEQ ID NO:28)          20                  25 and Cys-Gly Asn Tyr Arg Ala Tyr Ala Thr Glu Pro His Ala Lys Lys-NH(CH₂)—CO-Ala Met       1              5                   10                      15 Met Gin Ala Leu Gly Ala Arg Ala Lys Glu-NH₂ (SEQ ID NO:53).          20              25


5. Composition containing the biepitope compound according to claim 1 in solution in water, in plasma, or in a buffer solution.
 6. Composition according to claim 5, wherein the buffer solution is selected from the group consisting of phosphate, succinate, and Tris-HCl buffer solutions.
 7. Composition according to claim 5, wherein the buffer solution is selected from the group consisting of a succinate buffer solution pH=5-6 containing Kathon, Régilait, and EDTA, a succinate buffer solution pH=5-6 containing Kathon, casein, and EDTA, a phosphate buffer solution KH₂PO₄/K₂HPO₄ 0.1 M and pH=6.5-7.5 containing Kathon, Régilait and EDTA, and a phosphate buffer solution KH₂PO₄/K₂HOP₄ 0.1 M pH=6.5-7.5 containing Kathon, casein, and EDTA.
 8. A method for immunoassaying troponin I, wherein the biepitope compound according to claim 1 is used as a standard.
 9. The method according to claim 8, wherein the immunoassay is a sandwich immunoassay.
 10. A kit for carrying out an immunoassay comprising (a) the biepitope compound according to claim 1 or (b) a composition containing the biepitope compound in solution in water, in plasma, or in a buffer solution. 